The present invention relates to a reference mark identification system for use with a measuring system of the type which includes a measuring scale extending along a measuring direction, a measuring graduation defined by the scale, a plurality of reference marks, all of which define a single, preselected pattern along the measuring direction and each of which is positioned at a predetermined absolute position with respect to the graduation, and means for scanning the reference marks to generate reference pulses in response thereto.
In such a measuring system, reference control pulses generated at the reference marks can be used in various ways. For example, such reference pulses can be used to set the counter of the measuring system to zero in order to define a zero position of the measuring system. Alternately, such reference control pulses can be used to load a predetermined position value into the counter and to start the measuring process. Furthermore, such reference pulses can be used to control interference pulses as well as to act on a control arrangement coupled to the counter.
German Patent DE-PS 24 16 212 discloses an incremental length or angle measuring system in which a scale defines an incremental graduation and a plurality of reference marks on a separate track alongside the incremental graduation. The absolute values of these reference marks are determined from the different spacings between the individual reference marks. The spacings between the reference marks are determined by scanning the incremental graduation. Therefore, if the absolute position of any single reference mark is to be determined, two reference marks must be scanned. This process is relatively complicated and time consuming if for example two such reference marks lie far apart. Furthermore, in the event of a faulty or erroneous counting of the increments between two reference marks, the separation between the two reference marks can be measured inaccurately, and this can lead to false identification of the reference marks.
In German Patent DE-PS 29 52 106 there is described an incremental length or angle measuring system which includes a scale that defines both a measuring graduation and a plurality of reference marks situated alongside the graduation. In this system each of the reference marks is characterized by a unique line group distribution, different from all the other reference marks. The individual reference marks are scanned by scanning fields in a scanning unit and each reference mark has a particular scanning field allocated to it which defines the same line group distribution as the associated reference mark. This arrangement is relatively expensive, since the line group distributions of the individual reference marks should be distinguished from one another as strongly as possible in order to make possible an unambiguous identification of the individual reference marks. Furthermore, the scanning unit must include an identical scanning field for each of the reference marks to be identified.
German DE-OS No. 30 39 483 describes an incremental length or angle measuring system which defines a graduation track and a reference mark track arranged alongside the graduation track. A code mark track is arranged parallel to the graduation track and it includes code marks which identify respective ones of the reference marks. For the scanning of the reference marks and of the associated code marks, separate scanning fields on a scanning plate of a scanning unit are provided. Thus, the segments of the code marks are scanned by scanning fields which are provided particularly for these code mark segments and are different from the scanning fields used to scan the reference marks.